Radio transceivers commonly use voltage controlled oscillators (VCOs) and frequency synthesizers to generate various radio frequency (RF) signals for use as mixer injection signals. Wireless communication devices are increasingly required to cover a wide range of frequencies to allow operation in multiple regions of the world or in multiple operating standards, for example, EGSM and DCS bands in Europe and US Cellular and PCS bands in the United States. These requirements force VCOs to cover very wide tuning ranges, typically accomplished by the addition or subtraction of capacitance from the resonant tank circuit. At high capacitance values, the voltage swing across the resonant tank circuit drops, decreasing the signal-to-noise ratio of the oscillator, measured as phase noise. Traditionally, VCOs have been over-designed to meet the phase noise requirements under these conditions, leading to excess current drain or excessive inductor quality factor (Q) requirements.
In wideband transceiver applications, a VCO is typically required to tune across several frequency bands over process and temperature variations. This tuning is typically implemented by the addition or subtraction of capacitance from the resonant tank circuit. This addition or subtraction of capacitance causes a large variation in the impedance of the resonant tank circuit, and thus varies the loaded quality factor of the resonant tank circuit. Two important specifications for a VCO in a transceiver application are output voltage amplitude and single sideband phase noise. The quality factor of the VCO resonant tank circuit directly correlates to the VCO voltage amplitude and phase noise performance.
Traditionally, VCOs have been over-designed to meet the phase noise requirements under the worst case tuning conditions, leading to excess current drain or excessive inductor quality factor requirements. Excessive inductor quality factor requirements can lead to increased cost in the form of die area or process technology requirements.
Thus, there remains a need for a VCO capable of meeting the phase noise requirements under the worst case conditions while reducing overall current drain, thereby reducing cost and process technology requirements.